1. Field of Invention
The present invention relates to bottom layer, thermosetting anti-reflective coating compositions for use in deep ultraviolet multilayer photoresist systems, particularly to those having improved etch rates, conformality, and optical density at 248 nm wavelength exposure.
2. Background of the Prior Art
Bottom layer anti-reflective coating compositions for use in multilayer photoresist systems have traditionally contained high molecular weight thermoplastic binders such as polysulfones, polyurea sulfones, and poly(vinylpyridine) with high insolubility in photoresist solvents, which has also been referred to as "high differential solubility". These binders serve to inhibit intermixing of the anti-reflective composition with the top layer photoresist. Such thermoplastic binders often require strongly polar solvents such as N-methylpyrrolidone, .gamma.-butyrolactone, and tetrahydrofurfuryl alcohol, which are hygroscopic, have high surface tensions, and exhibit low volatility. While such solvents may be beneficial to "differential solubility", they also lead to a variety of film defects such as dimpling, dewetting, voiding, bubbling, and thickness variations, because of their low volatility.
There is a present trend to reduce the feature size of semiconductor circuitry. As the feature size approaches sub-0.30 micron dimensions, the aforementioned disadvantages of thermoplastic anti-reflective coatings, as well as the drawbacks described in U.S. Pat. Nos. 5,234,990, 5,401,614, 5,482,817, 5,554,485 and European patent application no. 95480087.6, incorporated herein by reference, become increasingly problematic. The most notable problem is that their so-called resistance to intermixing with photoresists becomes less and less complete. Accordingly, slight intermixing always occurs, producing small but discernable distortions at the bottom of resist features. Because the feature sizes are so small, even these small distortions become unacceptable for producing good quality, practical devices.
In order to overcome these drawbacks, there has arisen a need to develop binders, for anti-reflective coatings, from thermosetting, rather than thermoplastic, polymers. Such polymers would cure quickly enough to be coatable from fast-drying solvents, and thus solvent resistance and coating quality could be improved. U.S. Pat. No. 5,693,691, entitled Thermosetting Anti-Reflective Coatings Compositions, and our co-pending application Ser. No. 08/692,714, entitled Method for Making Multilayer Resist Structures with Thermosetting Anti-Reflective Coatings, herein incorporated by reference, describe the development of thermosetting anti-reflective coatings and disclose novel improvements in composition and methods. The anti-reflective coatings described therein are comprised principally of an oligomeric, hydroxyl-functional resin, an aminoplast crosslinking agent, a protonic acid catalyst, and an appropriate solvent vehicle, wherein the hydroxyl-functional resin is the reaction product of a phenolic or carboxylic acid dye with a low molecular weight epoxy resin having an epoxy functionality of 3 to 10. The coatings are cured by baking for 30 to 120 seconds at temperatures above 150.degree. C. As taught in U.S. Pat. No. 5,693,691 and our co-pending application, Ser. No. 08/692,714 these compositions, which are soluble in volatile organic solvents, are improvements over the prior art since they offer (1) high optical density in ultra thin films (&lt;2000 .ANG.); (2) virtually no intermixing with photoresists; (3) storage stability in catalyzed form; and (4) commercially feasible synthesis techniques for linking chromophores to an oligomeric resin matrix.
Although the above-described dye-attached thermosetting anti-reflective coatings derived from low molecular weight epoxy resins provide many unexpected benefits, they too have drawbacks. One such drawback occurs upon plasma etching images into the anti-reflective coating layer. For example, in U.S. Pat. No. 5,693,691 (see present, Comparative Example 1), oxygen plasma etching proceeds at rates no faster than about 1.25 times that of prior art thermoplastic resins, such as the polyarylethersulfone anti-reflective coating described in U.S. Pat. No. 5,234,990. Since polyarylethersulfone anti-reflective coatings are known to etch 1.25 times more slowly than deep ultraviolet photoresists, this implies that the thermosetting anti-reflective coating described in U.S. Pat. No. 5,693,691 will etch at approximately the same rate as the photoresist during the pattern transfer step. Since the anti-reflective coating layer thickness is typically 0.05-0.10 microns, a significant negative etch bias may be observed at resist feature sizes below 0.30 microns unless the plasma etch process is highly anisotropic.
Another limitation of dye-attached thermosetting anti-reflective coatings derived from low functionality epoxy resins is their tendency to planarize substrate topography rather than deposit conformally over surface features. The lack of conformality leads to even greater etch biasing of the photoresist since overetching must be applied to remove the anti-reflective coating from trench structures where it tends to build up during the coating and baking processes.
Dye-attached thermosetting anti-reflective coatings derived from higher molecular weight polymers than U.S. Pat. No. 5,693,691 have also been disclosed. For example, European patent application no. 92118070.9 describes deep ultraviolet anti-reflective coating compositions which contain a dye-attached acrylic copolymer and an aminoplast crosslinking agent. The copolymer is preferably formed by copolymerizing a 9-anthracene ester monomer such as 9-anthranol methacrylate and a hydroxyl-functional monomer such as 2-hydroxyethyl methacrylate (HEMA) with other methacrylate esters. The anthracene unit of the first monomer is the active dye, or chromophore, which imparts deep ultraviolet absorptivity to the copolymer. (See FIG. below.) HEMA provides a site for thermal crosslinking with the aminoplast reagent when the final coating is cured. ##STR1##
The anthracene chromophore is attached to the vinyl backbone of the copolymer via a carboxylic ester linkage. The short, stiff linkage between the chromophore and the copolymer backbone ultimately limits the content of anthracene-bearing acrylic units in the copolymer to significantly less than 100 mole percent, since at higher contents the copolymer becomes insoluble in preferred coating solvents. This, in turn, limits maximum film optical density and, correspondingly, the anti-reflection control power of the anti-reflective coating. Also, there is an absence of an adequate concentration of hydroxyl-functional comonomer to permit effective crosslinking of the copolymer. This further limits the optical density of the anti-reflective coating.
European patent application no. 94305124.3 discloses thermosetting anti-reflective coatings which comprise at least one compound (typically a polymer or oligomer) having one or more glycidyl functions, at least one phenolic anthracene dye, and a solvent capable of dissolving these compounds. However, unlike the anti-reflective coating compositions discussed above, the anthracene dye in the title compositions is not attached to the glycidyl-bearing polymer prior to cure and an aminoplast crosslinking agent is not present in the composition. Consequently, heating for several minutes at high temperatures is required to produce sufficient reaction between the phenolic anthracene dye and the glycidyl-bearing polymer to insolubilize the coating. This long cure cycle reduces wafer throughput and makes the process generally unacceptable to manufacturers. In addition, the preparation of the title anti-reflective coatings, particularly that of the phenolic anthracene dye components, involves many steps, making the coatings too expensive to produce and use on a practical basis.
U.S. Pat. No. 5,597,868 discloses thermosetting anti-reflective coatings for 193 nm photolithography which cure analogously to the coatings described in the just described European patent application 94305124.3. A polyphenolic dye such as a novolac resin is combined with an acrylic polymer which has pendant epoxide functionality. Heating the coating results in a thermosetting reaction between the phenolic hydroxyl groups of the dye and the epoxide groups of the polymer. As claimed therein, however, the curing process must proceed for more than 10 minutes at temperatures greater than 170.degree. C. to be effective.